The teachings of the present invention can be practiced on any magnetic material having domains of such size that refinement thereof would produce significant core loss improvement, such as amorphous materials, cube-on-face oriented electrical steel (designated (100) [001] by Miller's Indices) and cube-on-edge oriented silicon steels. For purposes of an exemplary showing the invention will be described in its application to improvements in the manufacture of cube-on-edge oriented electrical steel. In cube-on-edge oriented electrical steel, the body-centered cubes making up the grains or crystals are oriented in a cube-on-edge position, designated (110) [001] in accordance with Miller's indices.
Cube-on-edge oriented silicon steels are well known in the art and are commonly used in the manufacture of cores for transformers and the like. The teachings of the present invention will be described in terms of its application to cube-on-edge oriented silicon steels. It will be understood by one skilled in the art, however, that the teachings of the present invention are also applicable to magnetic materials wherein the domain size is suitably large that treatment in accordance with the present invention would be beneficial.
In recent years prior art workers have devised various routings for the manufacture of cube-on-edge oriented silicon steel which have resulted in markedly improved magnetic characteristics. As a consequence, such electrical steels are now considered to fall into two basic categories.
The first category is generally referred to as regular grain oriented silicon steel and is made by routings which normally produce a permeability at 796 A/m of less than 1870 with a core loss at 1.7 T and 60 Hz of greater than 0.700 W/lb when the strip thickness is about 0.295 mm.
The second category is generally referred to as high permeability grain oriented silicon steel and is made by routings which normally produce a permeability at 796 A/m of greater than 1870 with a core loss less than 0.700 W/lb (at 1.7 T and 60 Hz) when the strip thickness is about 0.295 mm.
U.S. Pat. No. 3,764,406 is typical of those which set forth routings for regular grain oriented silicon steel. For regular grain oriented silicon steel a typical melt composition by weight percent may be stated as follows:
C: less than 0.085% PA1 Si: 2%-4% PA1 S and/or Se: 0.015%-0.07% PA1 Mn: 0.02%-0.2% PA1 Si: 2%-4% PA1 C: less than 0.085% PA1 Al (acid-soluble): 0.01%-0.065% PA1 N: 0.003%-0.010% PA1 Mn: 0.03%-0.2% PA1 S: 0.015%-0.07%
The balance being iron and those impurities incident to the mode of manufacture.
In a typical but non-limiting routing, the melt may be cast into ingots and reduced to slabs or continuously cast in slab form. The ingots or slabs may be reheated to a temperature of about 1400.degree. C. and hot rolled to hot band thickness. The hot rolling step may be accomplished without reheating if the ingot or slab is at the required rolling temperature. The hot band is annealed at a temperature of about 980.degree. C. and pickled. Thereafter, the silicon steel may be cold rolled in one or more stages to final gauge and decarburized at a temperature of about 815.degree. C., for a time of about 3 minutes in a wet hydrogen atmosphere with a dew point of about 60.degree. C. The decarburized silicon steel is thereafter provided with an annealing separator, such as a coating of magnesia, and is subjected to a final high temperature box anneal in an atmosphere such as dry hydrogen at a temperature of about 1200.degree. C. to achieve the desired final orientation and magnetic characteristics.
U.S. Pat. Nos. 3,287,183; 3,636,579; 3,873,381 and 3,932,234 are typical of those teaching routings for high-permeability grain oriented silicon steel. A non-limiting, exemplary melt composition for such a silicon steel may be set forth as follows in weight percent:
The above list includes only the primary constituents; the melt may also contain minor amounts of copper, phosphorus, oxygen and those impurities incident to the mode of manufacture.
In an exemplary (but non-limiting) routing for such high-permeability grain oriented silicon steel the melt may be cast into ingots and rolled into slabs or continuously cast in slab form. The slab is reheated (if necessary) to a temperature of about 1400.degree. C. and is hot rolled to hot band thickness. After hot rolling, the steel band is continuously annealed at a temperature of from about 850.degree. C. to about 1200.degree. C. for from about 30 seconds to about 60 minutes in an atmosphere of combusted gas, nitrogen, air or inert gas. The strip is thereafter subjected to a slow cooling to a temperature of from about 850.degree. C. to about 980.degree. C., followed by quenching to ambient temperature. After descaling and pickling, the steel is cold rolled in one or more stages to final gauge, the final cold reduction being from about 65% to about 95%. Thereafter, the steel is continuously decarburized in wet hydrogen at a temperature of about 830.degree. C. for about 3 minutes at a dew point of about 60.degree. C. The decarburized silicon steel is provided with an annealing separator such as magnesia and is subjected to a final box anneal in an atmosphere of hydrogen at a temperature of about 1200.degree. C.
With respect to both types of grain oriented electrical steel, it is common practice, after the final high temperature anneal during which the desired (110) [001] texture is developed, to provide an insulative coating having a high dielectric strength on the grain oriented silicon steel (in lieu of or in addition to a mill glass) followed by a continuous anneal at a temperature of about 815.degree. C. for about three minutes in order to thermally flatten the steel strip and cure the insulative coating. Exemplary applied insulative coatings are taught in U.S. Pat. Nos. 3,948,786; 3,996,073; and 3,856,568.
The primary object of the present invention is to improve (i.e. reduce) the core loss of grain oriented silicon steel. Prior art workers have long addressed themselves to this problem and have devised both metallurgical and non-metallurgical means for reducing core loss. The metallurgical means include better orientation, thinner final thickness, higher volume resistivity and smaller secondary grain sizes. However, these metallurgical variables must be kept within prescribed limits to attain the optimum core loss in the finished grain oriented electrical steel. Maintaining this metallurgical balance has inhibited the development of materials with core losses closer to the theoretical limits. This has led a number of prior art workers to seek various non-metallurgical means to improve core loss after the metallurgical processing is substantially complete.
One non-metallugical approach is to apply a high-stress secondary coating onto the finished grain oriented electrical steel, as taught in U.S. Pat. No. 3,996,073. Such coatings place the grain oriented electrical steel strip in tension, which causes a decrease in the width of the 180.degree. magnetic domains and the reduction of the number of supplementary domains. Since narrow 180.degree. domains and few supplementary domains are desired in order to decrease the core loss of grain oriented electrical steels, such high-stress coatings are beneficial. However, the amount of tensile force that can be applied by these means is limited.
Another non-metallurgical approach is that of inducing controlled defects which is, in a sense, the creation of a substructure to limit the width of the 180.degree. domains in the finished grain oriented electrical steel. A basic technique is taught in U.S. Pat. No. 3,647,575 where the deformation of the surface of a grain oriented electrical steel is employed to induce defects to limit the length of the 180.degree. domains, resulting in a reduction in the width of the 180.degree. domains, thereby reducing the core loss. This is accomplished by providing narrowly spaced shallow grooves or scratches at or nearly transverse to the rolling direction and on opposite sides of the sheet. A grain oriented sheet treated according to U.S. Pat. No. 3,647,575 will have the insulative coating damaged and an uneven surface. These will result in increased interlaminar losses and decreased space factor, respectively, in a transformer fabricated from a steel so treated.
There have been numerous subsequent papers and patents which teach various methods of inducing controlled defects. Of particular interest are those techniques which limit domain size by localized irradition such as with a laser beam or the like, which, in part, overcomes the detrimental effects of the scratching process taught in U.S. Pat. No. 3,647,575.
USSR Pat. No. 653302 teaches that a grain oriented electrical steel can be treated with a laser beam to induce a substructure to regulate the domain wall spacing, thereby improving the core loss. According to USSR Pat. No. 653302, the surface of a grain oriented electrical steel sheet is irradiated at or nearly transverse to the rolling direction after the high temperature final anneal. In the irradiated regions, the sheet is rapidly heated from about 800.degree. C. to about 1200.degree. C. After the laser treatment, the grain oriented electrical steel sheet must be coated and annealed at a temperature of from about 700.degree. C. to about 1100.degree. C. A conventional grain oriented electrical steel sheet treated according to the teachings of USSR Pat. No. 653302 can have the core loss improved 10% or more; however, the permeability often is decreased and the exciting power increased, particularly when very thin final thicknesses of 0.30 mm or less are employed, which limits the commercial applicability of this technique.
U.S. Pat. No. 4,293,350 teaches another method of laser treatment for grain oriented electrical steel. According to U.S. Pat. No. 4,293,350, the surface of a grain oriented electrical steel sheet is briefly irradiated with a pulsing laser following the high temperature final anneal. The laser is so directed as to cross the surface of the sheet at or nearly transverse to the rolling direction. Irradiated regions are formed on the surface of the grain oriented silicon steel sheet within which a slight but significant substructure is induced to limit the width of the domains, thereby improving the core loss. Laser treatments done within the limits of U.S. Pat. No. 4,293,350 can improve the core loss of conventional grain oriented electrical steels by about 5% while high-permeability grain oriented electrical steels are improved by 10% or more without significantly degrading the permeability or exciting power of the treated sheet. The commercial applicability of an electrical steel treated according to these techniques is limited to stacked core transformer designs where the transformer core is not annealed to relieve the stresses resulting from fabrication. The slight laser-induced dislocation substructure will be removed upon annealing above from about 500.degree. C. to about 600.degree. C., while typical stress relief annealing is done at about 800.degree. C. An additional limitation of the technique taught in U.S. Pat. No. 4,293,350 is that the insulative coating, e.g., mill glass, secondary coating, or both, is damaged by treatment with a pulsed laser. Very high interlaminar resistivity and coating integrity is desired for grain oriented electrical steels used in stacked core designs.
European Pat. No. 33878 teaches a method of laser treating according to U.S. Pat. No. 4,293,350, followed by a coating operation and heating the laser treated and coated sheet to about 500.degree. C. to cure the coating; however, this technique necessitates additional processing steps and expense.
The present invention is based upon the discovery that grain oriented electrical steel having an insulative coating comprising a mill glass, an applied coating, or both, can be treated with a continuous wave laser to significantly reduce core loss through domain subdivision and refinement, without injury to the insulative coating, thereby resulting in a steel having laser refined magnetic domains with an uninterrupted coating.